This invention relates to an anisotropic-electroconductive adhesive composition for connecting circuits, an adhesive film made from such an adhesive composition, a method for connecting circuits by using these adhesive materials, and a connected circuit structure thus obtained.
As methods for connecting conductors arranged with very fine pitches such as connection of integrated circuits with wiring substrates, connection of display elements with wiring substrates, connection of electric circuits with leads, etc., there have widely been used a soldering method, and a method of using connecting materials such as an electroconductive adhesive, and the like. But, since the connecting materials should be formed on only electroconductive portions in these methods, the connection was very difficult in the case of fine circuits with higher density and higher fineness and sometimes adjacent conductors were undesirably connected with such connecting materials.
Recently, connecting materials for connecting conductors have been studied and disclosed in, for example, Japanese patent unexamined publication No. 55-104007 and U.S. Pat. No. 4,113,981, etc. Basic ideas of these publications reside in a process for using a so-called anisotropic-electroconductive connecting material wherein a layer of anisotropic-electroconductive connecting material including an electroconductive material such as metal particles is formed between conductors standing opposite to each other, electrical connection between the conductors and electrical insulating properties between adjacent conductors are simultaneously provided by applying pressure alone or pressure with heating, and the conductors standing opposite to each other are bonded and fixed.
But in these methods, since the electrical conduction between the conductors was obtained mainly by contact of a plurality of electroconductive materials such as metal particles in many cases, the contact areas between particles or between a particle and a conductor surface was insufficient due to rigidity of the metal particles and the reliability of conduction was insufficient.
On the other hand, in order to improve the initial connection reliability or to reduce the production cost using the same idea for electrical conduction as mentioned above, there have been proposed methods for obtaining anisotropic-electroconductive connection by using particles having electroconductive layers on the surfaces of electroconductive particles or insulating particles which were used as core materials (e.g. Japanese patent unexamined publication Nos. 56-122193, and 58-111202). But when metal particles, alumina, glass and the like were used as the core material, the contact areas between particles or a particle and a conductor surface were insufficient due to the rigidity of core materials and thus the reliability of conduction was unsatisfactory.
The above-mentioned prior art techniques make it possible to obtain anisotropic electroconductivity at the time of conductor connection, but cannot provide reliability at the connected portion for practical use. This is shown by Comparative Examples 1 to 4 in Table 1 mentioned below.
In order to improve the connection reliability, EP 147856, for example, proposes a process for connecting conductors by melting metal particles dispersed in an insulating adhesive between the conductors. But since the metal particles are melted with heating, the range for connecting conditions is narrow and thus there is a defect in that the reliability at the time of connection is unsatisfactory. That is, even if good connection can be obtained at near the melting point of a metal, but there is a defect in that metal particles flow and are deformed by melting at a temperature higher than the melting point like the conventional soldering to connect adjacent conductors, resulting in lose of insulating properties between adjacent conductors (being called "leak") and a lack of ability to be used for fine conductor pitches. Further, since the melting of the metal does not take place at a temperature lower than the melting point of the metal, the rigid metal particles are only present between conductors as mentioned above, thereby making it possible to obtain initial electroconductivity but, thereby making it unsatisfactory to obtain long connection reliability, particularly reliability, for example, in a thermal shock test. Further, it is difficult to control a melting state even at near the melting point from the viewpoint of thermal conductivity and the like. Even if good conductor connecting portions are obtained, since the difference in thermal expansion between an adhesive and the metal particles is large, there is a defect in that the thermal shock resistance is low as mentioned above.
In conductor connection for displays, one of major application fields of these anisotropic-electroconductive connecting materials, there have often been used transparent electroconductive films forming electroconductive circuits with thin layers of metal oxides such as tin oxides, indium oxide, titanium oxide, or aluminum, chromium, or the like on transparent substrates made from glass, plastics, and the like. But heat melting metal particles such as solder have poor wettability for conductor surfaces because of their extremely large surface tension and do not alloy an oxidation surface of aluminum or the like or oxide circuit, so that there are defects in that the wettability with the conductor surfaces is insufficient, and a changing rate of resistance to temperature change at connecting portions is large.
Thus, in the use for display devices such as liquid crystal display devices (LCD), electroluminescence (EL) plasma or fluorescence display tubes, there are problems for practical uses in that display at high temperatures becomes unclear, or the display becomes impossible. In order to improve such problems, a thin layer of Au or Ni is formed on the transparent electroconductive film by plating or sputtering so as to increase the surface tension of the conductors. But such a method requires complicated steps and higher treating techniques, so that the production cost of products is undesirably raised.